Satan - also known as Lucifer and/or The Devil - is the personification of evil in Abrahamic religion. This manifestation of negativity is said to have once been an angel that fell out of favour with God and tried to conquer Heaven, but was cast down into the depths of Hell. Though Hell was meant to be his prison, Satan made it his own kingdom and rules over the demons that dwell within.
Satan thrives on corruption and sin and has made it his purpose to plague mankind - God's favoured creation - by twisting human minds with temptation, which he has done so since the beginning of man when he took the form of a serpent and entered the Garden of Eden to tempt Adam and Eve with the fruit of the Tree of Knowledge.
Satan has been perceived in a variety of ways throughout history: a red-skinned man with horns and a pitchfork, a towering horned demon and a man with the legs of a goat and a serpent's tail, to name but a few. It could be that Satan has no "true" form and merely appears in any guise that a human mind imagines him to be. Given that he is temptation incarnate, he would surely take on a more seductive form that is pleasing to the eye.
Satan's powers are vast, but must surely have their limits if he is still confined to Hell. The full range of his powers is unknown, but it is generally accepted that he is capable of seeing into the minds and even souls of mortals and knows just how best to tempt men toward sin.